The CAN bus system has become widely accepted for the communication between sensors and control units. In the CAN bus system, messages are transmitted with the aid of the CAN protocol as it is described in the CAN specification in ISO 11898. Most recently, techniques have also been proposed in this regard, such as CAN-FD, in which messages are transmitted, and the like, in accordance with the specification “CAN with Flexible Data-Rate, Specification Version 1.0” (source http://www.semiconductors.bosch.de). Such techniques increase the maximally possible data rate through the use of higher clocking in the area of the data fields above a value of 1 Mbit/s. In general, this comes at the expense of the transmission quality, for example in the form of a higher bit error rate, provided that the process is based on actually present bus topologies.
Actually present bus topologies generally deviate from the theory to the effect that reflections arise on the bus line in locations at which the bus line has an impedance which differs from theory. Such locations are, for example, branchings, termination faults, mismatching or squeezed cables, which in practical implementations are often encountered in stubs, passive star points, and the like, for example. The resulting reflections cause the temporal crosstalk of states on the bus line in such a way that a transmitted symbol or bit affects the chronologically subsequent symbols by crosstalk and optionally distorts their detection.
According to the CAN specification in ISO 11898, the bus line should be terminated on both sides with the line impedance, so that the transient events for the specified maximum cable length decay within a transmitted symbol and a clear state results at the end of the symbol interval. In reality, however, crosstalk between two or multiple CAN symbols is often impossible to avoid.
The receiver of a CAN bus system is composed of a communication processor, which is usually integrated into a microcontroller, and a transmitter/receiver, which is also referred to as a transceiver and usually designed as a separate chip having a direct connection to the bus line. In such a transceiver, the reception path usually includes only one comparator having upstream voltage dividers for matching the bias of the bus levels. The comparator directly evaluates the bus levels of dominant and recessive bit states and makes a decision at the output.
However, the direct formation and output of signal decisions have the disadvantage that effects of reflections on the bus line negatively influence the decision and may result in incorrect decisions in the signal transmission. This is in particular the case with higher clocking in the area of data fields above a value of 1 Mbit/s, as applies to CAN-FD, for example. Reflections, which in a conventional CAN bus systems having a lower clock rate still constructively contribute to the decision, already have a negative impact here due to the shortened bit duration.
In general, reflections at cable transitions toward a higher impedance result in reflections having a positive sign. In contrast, reflections at cable transitions toward lower impedances result in reflections having a negative sign. Temporal shifts arise based on two reflections at a different distance.
The considered reflections drastically limit the field of use, for example with respect to possible topologies, cable lengths, and the like, of presently considered techniques having higher clocking of the data transmission, such as CAN-FD, and the like.
While equalization methods for improving the detection quality in the receiver are generally known in the field of communication technology, a use for CAN communication systems is not yet known. In addition, the use of known equalization methods for CAN communication systems necessitates special measures since these were not considered in the system design.